In case it is of interest for historical value, during the beta there was a period where you could only "respec" at a certain altar. There was an altar in town (near where the banners/portals appear), and at the start of at least one dungeon I remember.

I think the motivation was to preserve the idea of making you stick with a build and not be able to respec at will, without making it as laborious as in say WoW. However, the devs decided that in a game like D3, ultimately the flexibility to play however you want whenever you want contributed more to fun gameplay.